Kaolin clay, also referred to as china clay or hydrous kaolin, predominately comprises the mineral kaolinite (Al2Si2O5(OH)4), a hydrous aluminum silicate, and small amounts of various impurities, such as quartz, micas, smectite, graphite, iron oxides, iron sulfides, and/or titania-based impurities. Large deposits of kaolin clay exist in Brazil (e.g., in Rio Capim), in England (e.g., in Devon and Cornwall), in the United States (e.g., in Georgia and South Carolina), in Australia, and in several other countries. Kaolin is generally known as a white inorganic pigment existing naturally as and beneficiated from kaolin clay.
Although kaolin clay may be used as an inert filler where product specifications are not very rigid, a large quantity of kaolin clay is converted to kaolin products where restrictive specifications apply. Those specifications set limits on properties such as viscosity in water suspensions, particle size distribution, color, brightness, etc. The term “brightness” refers to the reflectance of the product to blue light. Since even the brightest commercial kaolins have a somewhat yellowish color, the blue reflectance (or brightness) is a reasonably good measure of their nearness to more perfectly white material. Impurities (sometimes called gangue), such as graphite, generally discolor the kaolin products and/or reduce their brightness. and ozone bleaching, that are used in the kaolin industry to remove discoloring impurities that result in improving the brightness of kaolin. The present disclosure generally relates to reverse flotation as a beneficiation method to remove discoloring impurities, such as graphite, from kaolin clays.
Flotation is a process of treating a mixture of finely divided mineral solids, such as a kaolin clay, suspended in a liquid to separate a portion of the solids from other finely divided mineral solids, such as impurities. In general, flotation comprises introducing a gas (or providing a gas in situ) into the liquid to produce a frothy mass containing certain of the solids on the top of the liquid, and leaving suspended (unfrothed) other solid components. Flotation is based on the general principle that introducing a gas into a liquid containing suspended solid particles of different materials causes adherence of some gas to certain suspended solids and not to others, making the particles having the gas adhered thereto “lighter than” the liquid and allowing them to rise to the top of the liquid to form a froth. Those minerals remaining at the bottom or within the liquid are generally referred to as the “tailings.” Some minerals and associated gangue treated by flotation generally do not possess sufficient hydrophobicity or hydrophilicity to allow adequate separation using gas alone. Various chemical reagents are often employed in froth flotation to create or enhance the physical or chemical properties necessary to cause separation.
As one such example of a chemical reagent, collectors are generally used to enhance the hydrophobicity, and thus floatability, of different mineral values. Collectors should be able to (1) attach to the desired mineral species to the relative exclusion of other species present; (2) maintain attachment during turbulence, shear, or other forces associated with froth flotation; and (3) render the desired mineral species sufficiently hydrophobic to permit the desired degree of separation.
Additional chemical reagents may include depressants, pH regulators (such as lime and soda), dispersants, and various promoters and activators. Depressants are generally used to increase or enhance the hydrophilicity of various mineral species and, thus, depress their flotation. Promoters and activators increase or enhance the effectiveness of other reagents, such as collectors or depressants.
Froth flotation has been extensively practiced in the mining industry since at least the early twentieth century. In the typical or direct flotation scheme, the valuable or desired mineral(s) is floated away from the gangue, which is left in the tailings. In another flotation scheme known as reverse flotation, the undesired gangue is floated away from the valuable mineral(s), which are left in the tailings. As one example, sedimentary kaolin clays have been subjected to flotation processes to remove anatase, a titania-based impurity. See J. Miller, B. Tippin, and R. Pruett, Nonsulfide Flotation Technology and Plant Practice, in 1 Mineral Processing Plant Design, Practice and Control 1159 (A. L. Mular et al. eds., 2002).
Certain flotation processes used to produce kaolins of various brightness have been discussed in the art. For example, U.S. Pat. No. 5,883,029 appears to disclose improving the brightness of various ores, including kaolin, by atomization or direct fusion. U.S. Patent Publication Nos. 2008/0017552 and 2008/0029460 likewise appear to disclose modified resins for removing various impurities from mineral and metal ores using froth flotation. Sharad Mathur, “Kaolin Flotation,” 256 J. Colloid & Interface Sci. 153 (2002) also discloses certain kaolin flotation methods. However, none of these references appear to disclose removing carbon impurities, such as graphite, from kaolin by reverse flotation.
High brightness kaolin clay has many uses in industrial applications. For example, deficiencies of barrier coating due to the presence of graphite may be decreased or eliminated by removing graphite impurities. Other industrial applications include, but are not limited to, using high brightness kaolin clays as a coating pigment in paper applications and as a filler in paper, paints, and ceramics.